The Global System for Mobile communication (GSM) describes a European standard for radiocommunication utilized by the corresponding Public Land Mobile Networks (PLMNs) in the region and in many other countries, which standard is intended to provide uniformity so that users can access radiocommunication systems throughout Europe and many other countries with minimal equipment compatibility problems. In addition to lower level details such as modulation, frame formats, etc., the GSM standard also specifies standard higher level activities which can be performed within the system. There are many such activities which may be associated with a particular subscriber in the GSM network including call related events such as call set-up, and call termination. Other types of subscriber activities include invocation of call related and call independent supplementary services such as call hold, call waiting, call transfer, and call forwarding. Based upon roaming agreements between the different mobile network operators, mobile subscribers belonging to a specific (home) PLMN, referred to herein as an HPLMN, can make use of their subscribed services and facilities while roaming on other PLMNs, referred to as the visited PLMN (VPLMN).
FIG. 1 illustrates the network architecture used in GSM systems. Home location register (HLR) 115 stores data relating to subscribers including, for example, current location of the subscriber equipment, directory number (MSISDN), radio number plan identification (e.g., International Mobile Subscriber Identity (IMSI)), supplementary service profiles and teleservice profiles. Visitor location register (VLR) 120 manages data associated with subscribers which are currently situated within its area of responsibility, e.g., those mobiles which have roamed into its service area. Authentication Center (AUC) 145 works in close association with HLR 115, wherein AUC 145 provides the information to authenticate all calls in order to guard against possible fraud, stolen subscriber cards or unpaid bills.
When mobile unit 150, a subscriber of HPLMN 101, tries to invoke a subscriber activity while in PLMN 102, VLR 120 requests a xe2x80x9ctripletxe2x80x9d of information from HLR 115 in order to determine whether mobile unit 150 is authorized to use the particular service. The xe2x80x9ctripletxe2x80x9d consists of a random number (RAND), a signed response (SRES) and a key cipher (Kc), which are generated by AUC 145. The xe2x80x9ctripletxe2x80x9d of information is requested by HLR 115 and stored therein. The xe2x80x9ctripletxe2x80x9d of information is transmitted to VLR 120 from HLR 115 in response to a specific request from VLR 120, e.g., during location updating. VLR 120 transmits the RAND to mobile unit 150. In response mobile unit 150 performs a calculation based upon the RAND in order to determine a SRES which is transmitted to VLR 120. VLR 120 compares the SRES received from HLR 115 and the SRES received from mobile unit 150. If the SRESs match then VLR 120 determines that mobile unit 150 is authorized. HLR 115 updates its database to indicate that mobile unit 150 is located in VLR 120.
When mobile unit 150 moves from PLMN 102 to PLMN 190, mobile unit 150 recognizes the change and sends a location updating message to the new VLR (not shown) in PLMN 190. The new VLR informs HLR 115 of the new location of mobile unit 150. HLR 115 then sends a Cancel Location message to previously visited VLR 120, located in PLMN 102, to indicate that mobile unit 150 is no longer present in the area served by PLMN 102. HLR 115 sends an insert subscriber data (ISD) message to the new VLR which provides the new VLR with the relevant subscriber data.
This architecture supports simultaneous activities in different VLRs in case of subscriber movement. In addition, activities on different gateway mobile switching centers (GMSCs) may be maintained simultaneously in certain service scenarios. One scenario in which there are simultaneous subscriber activities is support for optimal routing, a network feature which enables the calls directed to a mobile subscriber to be routed directly to the mobile subscriber""s actual location, or to her forwarded-to destination (instead of via the HPLMN or in the case of Late Call Forwarding via the VPLMN). For instance, in an exemplary optimal routing scenario, an HLR may receive Send Routing Information (SRI) messages from more than one GMSC. The PLMN associated with a GMSC in an optimal routing scenario is referred to as an interrogating PLMN (IPLMN). The IPLMN interrogates the HPLMN of a called party in order to determine the whereabouts of that party. The interested reader may refer to GSM standard documents numbered 02.79 and 03.79 for more details of the optimal routing feature.
Allowing simultaneous subscriber activities in GSM make certain fraud scenarios very difficult to control by the network operator due to the fact that a legitimate subscriber may invoke several subscriber activities in a single network or in different networks. It is possible for a subscriber to enter an area served by a particular MSC/VLR while still having subscriber activities which are ongoing in a VLR associated with a previously visited MSC/VLR area which the subscriber has left. For instance, referring again to FIG. 1, mobile unit 150 may initiate a call forwarding service in GMSC 170 while located in the area served by PLMN 102. Mobile unit 150 may then move into the area served by PLMN 190 and invoke another call service, without canceling the call forwarding service activated while mobile unit 150 was located in the area served by PLMN 102. Therefore, mobile unit 150 will have activities in both PLMN 102 and PLMN 190 which appear to an operator of a PLMN as fraudulent subscriber activity.
An HPLMN may control fraudulent subscriber activity using an Immediate Service Termination (IST) command to terminate the fraudulent activity. FIG. 2 illustrates the implementation of the IST command. In step 205, the HPLMN changes the subscriber""s entry in the HLR, e.g. by removing the subscribers subscription, in order to prevent the subscriber from resuming activities in the HPLMN and VPLMN after the invocation of the IST command. In step 210, the HPLMN sends a Mobile Application Part (MAP) Cancel Location command to the VLR in order to prevent the resumption of activity by the subscriber within the VPLMN without the need to reference the HPLMN. In step 215, the HPLMN sends an IST command to the VPLMN. In step 220, the VPLMN confirms receipt and comprehension of the IST command. In step 225, the VPLMN terminates all activities of the subscriber which were indicated by the IST command. In step 230, the VPLMN sends confirmation to the HPLMN that all activities have been terminated.
As stated above, a subscriber may have remaining activities in PLMNs where the subscriber is no longer located. In order for the IST command to be truly effective, all subscriber activity should be terminated. Hence, there is a need for the HLR to track subscriber activity not only in the current VPLMN, but also in all PLMNs in which the subscriber has remaining activities. There are several proposed solutions which attempt to address this issue.
One solution is based upon custom applications for mobile network enhanced logic (CAMEL). CAMEL provides international intelligent network services for the GSM. CAMEL can be used to terminate all originated, terminated and forwarded calls of a subscriber, provided there is a control relationship between the CAMEL server in the HPLMN and the MSC controlling the call or forwarding leg. Using CAMEL requires the service control point (gsmSCP) to control all ongoing calls. As the amount of subscribers who are using CAMEL increases, the capacity required of the gsmSCP and the service switching point (gsmSSP) likewise increase which leads to a greater expense in monitoring the remaining activities. Normally, when a subscriber has been removed from the HLR it shall be communicated to the SCP to start the IST procedure. However, such an operation cannot occur through the use of CAMEL because the standard fails to define any coordination between the gsmSCP data and the HLR data.
Another solution is based upon subscriber activity reporting from the VLR to the HLR. In the GMSC a timer is setup on a per call basis at the start of any call forwarding. An IST alert is then sent when the call is active longer than the timer limit. The sending of the IST alert is repeated upon each occurrence of the expiration of the timer. However, upon the expiration of the timer the call is not released. In a visited MSC (VMSC), the timer starts after cancellation of the data in the VLR, if the subscriber still has remaining activities. Similar to the description above in regard to the GMSC, an IST alert is sent to the HLR upon each expiration of the timer. If the HLR determines that it is necessary to terminate all of the activities of a subscriber, the HLR can send to the previously visited MSC/VLRs or to the GMSC an IST command upon reception of the IST alert, in order to stop all remaining activities.
This solution has many drawbacks, including the large network capacity which is required in the MSC/VLR because of the need to set a timer for all calls, as compared to setting timers for only the most risky subscribers and PLMNs. Since the timer in the MSC/VLR is not transmitted in the network, the HPLMN does not know how much time to wait for incoming IST alert messages because such messages are controlled by the VPLMNs. According to this solution the timer is fixed regardless of the fraud risk posed by a particular subscriber.
These and other drawbacks and difficulties found in conventional radio communication systems, for example the GSM system, are overcome according to the present invention. According to exemplary embodiments of the present invention, the HPLMN does not control the ongoing activities in the VPLMN, nor in the IPLMN. The HPLMN is informed of remaining activities by the VPLMN and the IPLMN during certain time periods. For example, reporting to the HPLMN starts when a Cancel Location command is invoked or Send Routing Information command is answered. The period for notification regarding ongoing activities is set by the HPLMN in the VPLMN and the IPLMN on a per subscriber basis, based upon the fraud risk posed by a particular subscriber. Alternatively, the period for notification may be set based upon the risk associated with the particular PLMN which is controlling the subscriber activity. Various time periods may be set depending upon this risk. According to an exemplary embodiment of the present invention the time periods may be set based upon a combination of the risk posed by a particular subscriber and the risk posed by a particular PLMN. Before issuing an IST command to a GMSC or an previously visited MSC/VLR, the HPLMN waits for notification via an IST alert of the ongoing activities based on the amount of time assigned per subscriber. According to an exemplary embodiment of the present invention, the GMSC or MSC/VLR can indicate to the HLR whether service termination is supported by the node.